One of the primary goals of comet observations is to understand the composition of their nuclei, from which we hope to obtain significant constraints on the conditions in the early solar system during the time of planet formation. Key questions are how cometary material is formed, to what extent interstellar matter from the pre-solar cloud might be preserved, and the amount of later processing since formation. Observations of comets with groundbased telescopes, or telescopes in orbit around Earth, are unable to resolve the small cometary nucleus hidden in the gaseous coma and dust tail. Any information on its composition and internal structure can, therefore, only be derived by indirect means from observations of its neutral and ionic emission features, and the scattered solar light on dust particles.
The nucleus itself might not be a homogeneous body. Chemical and/or physical inhomogeneities might be present as a result of formation conditions. Differentiation processes within the nucleus can evolve during subsequent orbital passages. Compositional differences might exist among populations of comets depending on their formation conditions or later processing. The nucleus composition and structure needs to be studied in a statistically significant sample of comets if general conclusions on their formation are to be drawn. Remote observations can provide the comparative analysis of a large number of comets and, therefore, ideally complement the detailed in situ measurements of space probes, which are possible only for a few objects.
The main volatile constituent of the nucleus is water ice, followed by carbon monoxide. As a result of its sublimation temperature of 152 K, water sublimation is expected to be significant only for comets within 3-5 AU from the sun. Sublimation in comets at larger distances is dominated by CO which has a sublimation temperature of only 24 K. Although the activity of comets is determined by these two main volatiles, the minor species with abundances of at most a few percent give important clues for the understanding of the origin and formation of comets. Spectroscopic observations provide a list of species emitting in the coma. Unfortunately, the conversion of coma abundance ratios to nucleus composition is not straightforward, but depends on the outgassing processes. Therefore models simulating the sublimation process are required to quantitatively link the coma to its origin, the nucleus. Monitoring of the activity of many gaseous species over a wide range of heliocentric distances provides significant constraints on such models.
With observations from groundbased and orbiting telescopes one can obtain information on the cometary nucleus by studying the cometary dust and gas coma. Important information on the physical structure of the nucleus and the outgassing processes at the surface can be obtained by measuring the variation of cometary activity along the orbit of a comet. The exceptionally bright comet Hale-Bopp, discovered in 1995, allowed such a study to be performed for the first time over a wide range of heliocentric distances.
An optical monitoring program of Hale-Bopp's activity (PI: Heike Rauer) was started in 1996 to study the comet's activity on its way towards the Sun from 4.6 AU to 2.9 AU and on his path outward from 2.5 AU to more than 6 AU. The observations were made at the European Southern Observatory (ESO), Chile. Medium-resolution spectra were taken covering a range from 380 nm to 700 nm. These spectra show emission from the coma daughter species CN, C3, C2, NH2, CO+ and H2O+. The evolution of their activity with heliocentric distance has been studied. The detected species provide information on their parent molecules, such as HCN, NH3, CO and H2O. In addition, the long-slit spectra allow dissociation rates of the cometary parent and daughter molecules to be verified, even at large heliocentric distances where they are poorly known, and the excitation mechanism of the cometary daughter species to be studied. Furthermore, images of the cometary dust coma allow the dust production rate and the morphology of the dust coma to be studied. The long-term monitoring program of comet Hale-Bopp has been continued at ESO, starting in September 1997, to follow comet Hale-Bopp on its way out of the inner solar system. The observation will continue till mid of 1999, when the comet will be too faint for spectral measurements. The observations will show, for example, any asymmetries in activity with respect to perihelion and dependance of the activity of the different species on heliocentric distance.
The activity of daughter species observed in the optical domain has been compared to observations of their parent molecules which can be directly measured at mm and sub-mm wavelengths. A monitoring program at these wavelengths is being performed at the IRAM 30 m telescope, Spain, by the working group on cometary radio emissions at the Observatoire de Paris-Meudon (PI: J. Crovisier). The program started in September 1995 and is continuing at the SEST telescope at ESO to follow the comet on its outbound path. A comparison of optical and radio emissions is made in collaboration with the group in Meudon.
Observations of the cometary activity evolution with heliocentric distance have been compared with results from modeling the sublimation processes in the surface of the nucleus. Models simulate the sublimation of water molecules under the influence of the varying energy input along the orbit including the influence of heat conduction in the nucleus.
To observe comet Hale-Bopp during its perihelion passage in spring 1997 a coordinated campaign involving 19 scientists was set up in the context of an international time proposal at the observatories on the Canary Islands, Spain (PI: R. West). Imaging and spectroscopic observations were performed in the optical and infrared regions from March to May 1997.
Heike Rauer and Claudia Lemme participated mainly in the program involving medium- and high-resolution spectroscopy. An additional spectroscopic observing campaign (PI: C. Arpigny), again including medium and high-resolution observations, was carried out at the Observatoire de Haute Provence, France during March and April 1997. The spectroscopic observations allow the neutral daughter species (NH, NH2, CN, C2, C3, etc.) to be studied and various ions, such as CO+, OH+, and H2O+, to be monitored.
A surprising result of the imaging observations performed by the European Hale-Bopp team at the Canary Islands was the discovery of an extended sodium tail in comet Hale-Bopp. Analysis of the spectroscopic sodium observations made of the coma of Hale-Bopp allow the spatial distribution and kinematics of sodium atoms to be investigated. Reduction and analysis of the spectroscopic data taken at the Canary Islands and the Observatoire de Haute Provence are currently in progress.
Observations of cometary ions can reveal information on the interaction of the comet with the solar wind. In addition, ions play an important role in the chemistry of the inner cometary coma. The two main ions in cometary plasma tails are CO+ and H2O+. CO+ is frequently observed by means of its emission bands in the blue optical range. However, it also has emission bands in the radio region. To estimate if these bands could be detected a model of resonance fluorescence excitation has been developed by H. Rauer and colleagues which includes the lowest rotational levels of the electronic ground state. Comparisons of the model with observations of CO+ are in progress.
Emissions of CO+, HCO+ and H3O+ have been detected for the first time during the monitoring of Hale-Bopp at radio frequencies (see above). These are the first observations of ionic emission in comets at radio frequencies. The ion HCO+ is frequently seen in the interstellar medium, but only now has it been detected in comets. The reduction and analysis of these ion observations are in progress.
A problem of cometary physics which has attracted much attention in recent years is the possible existence and evolution of a cloud of large particles (mm- to m- sized) around cometary nuclei. This motived E. Kührt and J. Knollenberg to develop a numerical model for the large particle dynamics, taking into account gravity, inertia, tidal, solar pressure and gas drag forces. The gas drag is modelled by a gasdynamic solution of the Euler equations in 2D-axialsymmetric geometry, thus using a more realistic assumption for this major force component than other models previously published. The calculations show that a fraction of 10-5 to 10-3 of the ejected particle mass (dependent on the dust size distribution) can stay in bound trajectories for more than 3 months. Furthermore, a few percent of the particle mass is falling back onto the nucleus thereby creating a dust layer of a few cm thickness during one orbital revolution (Fig. 4).
Returned mass flux of ejected dust integrated from 3 AU to perihelion
for a comet with 1 km radius. Eight active regions are randomly
distributed on the cometary surface and their position is
indicated by the white dots.
When a comet approaches the Sun volatile material in the surface layer is sublimated and accelerated. The resulting gas flow may also accelerate dust particles and macroscopic pieces of the surface. Due to the extremely low gravity at a comet's surface an extended cometary atmosphere (the "coma") is produced. Interaction with the solar wind draws the coma into a tail which may extend millions of kilometeres.
The processes which take place near a porous cometary surface are being investigated by U. Motschmann and colleagues using gas and dust kinetic models. To describe the extremely low densities adequately a particle-in-cell simulation is applied. In the present approach the model incorporates the interaction of the particles with the walls of the pores in the surface layer and collisions with each other. The simulation provides the distributions of the gas and dust components, their densities, drift velocities, accelerations etc., as functions of the solar radiation and the thermal and structural properties of the cometary material. Monte Carlo simulations are planned to determine the drag coefficient for the acceleration of dust particles in a gas stream. In contrast to the classic Probstein approach, nonspherical particles and gas-gas collisions are included.
Starting from a thermal model that includes an ellipsoidal shape of the nucleus and its rotation, E. Kührt has calculated the H2O activity along the orbit of P/Wirtanen and Hale-Bopp. Local activity was modelled by solving the energy balance at the surface, including heat transport. The cometary water production is derived by integrating local rates over the whole nucleus. Comparison of the results with observations enabled the thermal conductivity of the cometary material to be estimated.
Within the Working Group IV (Cometary Nucleus) of the ISSI (International Space Science Institute in Bern) structural properties of cometary nuclei, such as density, mechanical strength, and dust-to-ice ratio, have been reviewed and a preliminary report issued.
Landing on a cometary nucleus, as planned during the ROSETTA-mission, is of high scientific interest and will substantially increase our knowledge of the nature and origin of comets. However, successfully landing and operating the instruments is an extraordinary challenge to engineers and scientists.
In an extensive study by E. Kührt and colleagues (Kührt et al., 1997) a wide range of uncommon and risky environmental conditions such as outgassing, a fragile surface, deposition of dust grains, extreme temperature variations, and pronounced topography were considered. These are described in physical terms based on model calculations ranging from simple one-dimensional heat transfer to gas-dynamic acceleration of dust particles. The drag forces of the gas molecules on the lander have been calculated and identified as a potential risk even at large heliocentric distances.
Ratio of gas drag force to gravity imposed on the Rosetta S/C by a gas jet evaporating from a circular active region of 0.38 km2 size at a heliocentric distance of 3 AU. The assumed nucleus radius is 1 km and it's bulk density 500 kg m-3 and the axis of the gas jet coincides with the X-axis. It is evident that the gas drag can have significant influence on the S/C trajectory.
Asteroids and Comets Section home page
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|Author: Asteroids and Comets Section, WWW-Author: Dr. Alan Harris|
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